TUESDAY, Aug. 20 (HealthDay News) -- By the time they reach the fourth grade, children exposed to lead are nearly three times more likely to have been suspended, a new study contends.

The findings from nearly 4,000 children in the Milwaukee school district suggest that lead exposure may play more of a role in school discipline problems than was realized, according to the University of Wisconsin-Madison researchers.

"Students who are suspended from school are at greater risk of dropping out, twice as likely to use tobacco, and more likely to engage in violent behavior later in life," study first author Michael Amato, a doctoral candidate in psychology and at the Nelson Institute for Environmental Studies, said in a university news release.

Black students are three times more likely to be suspended than white students nationally, according to background information in the news release. The same difference was found in this study, but differences in rates of lead exposure accounted for 23 percent of the disparity, the researchers said.

Black children are more than twice as likely as white children to have elevated lead levels, according to the U.S. Centers for Disease Control and Prevention. Researchers attribute this to black children being more likely to live in low-income neighborhoods and rental housing where lead remains in the buildings and soil.

Many previous studies have identified disparities in school discipline, but few have pinpointed the underlying factors, the news release said.

"We knew that lead exposure decreases children's abilities to control their attention and behavior, but we were still surprised that exposed children were so much more likely to be suspended," study co-author Sheryl Magzamen, who is now an assistant professor at the University of Oklahoma, said in the news release.

The researchers noted that animal experiments have shown that lead causes decreased attention and decreased control over behavior when an animal is startled or touched. If children exposed to lead behave the same way, they're more likely to have disruptive classroom behaviors that can result in suspension, according to the researchers.

Although the study found an association between childhood lead exposure and increased risk of school suspension, it didn't prove cause-and-effect.

The study appears in the September issue of the journal Environmental Research.